INTEGRATED PETROPHYSICAL FORMATION EVALUATION USING CAPTURE SPECTROSCOPY AND NMR ON AN EXPLORATION WELL, SOUTH SUMATRA

摘要

This paper presents an integrated petrophysical formation evolution on two complicated geology settings penetrated by an exploration well in South Sumatra region. The rich information from the capture spectroscopy and nuclear magnetic resonance provides tremendous applications on fluid typing on the low resistivity contrast sandstones in a fresh formation water environment; and the lithology identification plus other reservoir properties characterization on an unconventional reservoir. The interpretation guided the selection of test candidates and has been confirmed by the test results.rnThe first formation is a clastic sequence where fluid typing was the main challenge. On conventional logs most of the sands have similar petrophysical characters, in terms of the resistivity reading, lack of Density-Neutron cross over, etc. In addition, a dubious clay volume derived from gamma ray and fresh formation water made the interpretation even more challenging.rnIn view of these difficulties, The Company decided to acquire capture spectroscopy and NMR logs in order to reach a solid conclusion for the first exploration well in this structure. All hydrocarbon-bearing zones were clearly identified. The sedimentary sequence for the main gas-bearing sand was clearly revealed by the spectroscopy-NMR combination; for another zone, the spectroscopy log showed a much lower clay volume compared to the traditional GR estimation, and the Density-NMR combination revealed this zone to be gas bearing, even though there was no Density-Neutron cross-over. A gas cap was identified on therntop of massive water-bearing sand, raising expectation that more gas can be found higher in the structure for the same sand.rnAt the bottom of the same well, a very radioactive massive formation was discovered, the natural gamma ray spectrometry showed that both thorium and potassium readings are very high; neutron and density logs cannot provide conclusive lithology and porosity as well. Capture spectroscopy identified one clean sand at the top of the formation. The permeability from NMR is also high. Very long T2 relaxation times are suggesting that there is big potential of existing very light oil.rnThe detailed mineralogical information derived from the comprehensive measurements of gamma ray capture spectroscopy and NMR implies that the sedimentary environment for this formation is most probably an alluvial deposition and the source rock is weathered granite. This log-based conclusion helped in calibrating the existing reservoir model.